Transcend 32GB USB slow transfer - USB 2.0 becomes 1.1?

The problem: transfer rates with my USB drive, normally 14-16 MB/s, sometimes drop to ~1 MB/s. When the speeds slow, checking the drive properties suggests the drive is connected to a USB 1.1 port. The speeds are a bit of an issue as I need to transfer outlook pst's (~4-5 GB at a time) to and from the USB drive each day - normally it is a few minutes, but when speeds slow it can take >1 hr.

Strangely, while one drive may seem to be connected to a USB 1.1 port, another drive connected at the same time reports it is connected to a USB 2.0 port. Equally strangely, the reported speed a port has will change (today the middle port is stuck at USB 1.1 speeds; tomorrow the middle port is OK but the far left port is stuck at USB 1.1. speeds).

I am running Win 7 ultiimate x64 on a Dell Latitude D6420 with 3 USB 2.0 ports and an eSATA port. I am using Transcend 32GB and 64B USB sticks ("ultrafast" = V600). The appropriate drivers from Dell are all installed and Device Manager shows no problems with the USB drives. All drivers are up to date according to Windows. One of the USB drives is formatted as exFAT and the other as NTFS. Connecting the USB stick to the eSATA / USB port does not ever seem to give me USB 1.1 transfer speeds.

Some earlier posts on the subject suggest some ideas (update drivers, format as NTFS). None seem very applicable to my problem. For example:
8gb Transcend Flash Drive with slow read/write speeds
Slow USB transfer

THANKS for any suggestions.

The PST files are big - 100s of MB to 1 GB. I understand the issue of file size - the problem is, given the same files being transferred, why do my USB ports drop to slow speeds, when they can support much faster transfer rates?
Thanks again